1. Field of the Invention
This invention relates to a biphenyl derivative, and an organic light-emitting element using the biphenyl derivative.
2. Description of the Related Art
In recent years, dramatic progress has been made in organic light-emitting elements. As characteristic features thereof, it is noted that high luminance at low applied voltage, a variety of light emission wavelengths and quick response can be achieved, and light-emitting devices can be made thin and light-weight. Accordingly, such organic light-emitting elements are indicating possibilities for a wide range of uses.
Fluorescent organic compounds are contained in electron transport layers, light-emitting layers, etc. the organic light-emitting elements include. Such fluorescent organic compounds include aromatic compounds and condensed polycyclic aromatic compounds, and many studies have been made on these compounds. However, when fluorescent organic compounds having hitherto been proposed are used for organic light-emitting elements, it cannot be said that light-emitting elements having satisfactory light emission luminance and durability have been obtained.
Studies are recently being made not only on conventional light-emitting elements utilizing the fluorescence that is produced at the time of transition from the excited singlet state to the ground state, but also on organic light-emitting elements utilizing the light emission by phosphorescence via next triplet excitons. As specific examples, the following Non-patent Documents 1 and 2 may be given. In these specific examples, an organic light-emitting element is primarily used which is made up of a layer formed of organic compounds in four layers. Specifically, this organic light-emitting element has a hole transport layer, a light-emitting layer, an exciton diffusion preventive layer and an electron transport layer in this order from the anode side.
A compound is proposed which has as its basic skeleton biphenyl into which an aromatic compound or a heterocyclic compound has been introduced, and has been applied to organic light-emitting elements. As specific examples, the following Patent Documents 1 to 3 may be given.    Patent Document 1: WO2004/074399.    Patent Document 2: WO2005/063920.    Patent Document 3: US2006/0029829.    Non-patent Document 1: Improved energy transfer in electrophosphorescent device (D. F. O'Brien et al., Applied Physics Letters, Vol. 74, No. 3, p. 442 (1999).    Non-patent Document 2: Very high-efficiency green organic light-emitting devices based on electrophosphorescence (M. A. Baldo et al., Applied Physics Letters, Vol. 75, No. 1, p. 4 (1999).
In order to apply organic light-emitting elements to display devices, the organic light-emitting elements are required to have highly efficient and highly luminescent optical power and at the same time to ensure high durability. However, any organic light-emitting elements having hitherto been proposed cannot still be said to satisfactorily have resolved these problems.